Boron-containing compounds, and particularly borate esters are known to act as antiwear agents when added to lubricating oils. EP-A-0216909 discloses antiwear agents that are esters of metaboric acid, and have the following formula: ##STR3## wherein each R is independently hydrogen or a hydrocarbyl group containing from 1 to 18 carbon atoms and each R' is independently an alkylene group containing from 2 to 4 carbon atoms.
It is further well known that sulfur-containing compounds act as anti-oxidants in lubricating compositions and can further enhance the effect of boron-based antiwear agents. The above described European Patent Specification discloses the use of the defined metaboric acid ester in combination with an oil soluble sulfurized organic compound in relative amounts sufficient to provide a weight ratio of sulfur to boron of from 0.5:1 to 20:1.
Antiwear agents providing both boron and sulfur are disclosed, for example, U.S. Pat. No. 3,303,130 describes an organo thioalkyl borate antiwear agent of the general formula: ##STR4## wherein R is selected from the group consisting of hydrogen, alkyl, aryl, alkaryl, aralkyl and cycloalkyl radicals containing 1 to 16 carbon atoms and n is an integer of 2 to 16, inclusive. These compounds are formed by reacting a thioalcohol with boric acid in a molar ratio of at least 3:1, and provide an antiwear additive having a weight ratio of sulfur to boron of 3.33:1. Similar compounds formed by reacting an alcohol, a hydroxysulfide and a boron compound, and the use thereof as a friction reducer in lubricating oil compositions are disclosed in U.S. Pat. No. 4,492,640.
Because of increased demand for lubricating oil additives and fierce competition between manufacturers, there has been a continued need for improved antiwear additives. The present inventors have developed an improved antiwear additive for lubricating oils which comprises a single compound that provides a relatively high weight ratio of boron to sulfur, while simultaneously providing antioxidant and friction modifier properties.
Other objects, advantages and features of the present invention will be understood by reference to the following specification.